1. Field of the Invention
The present invention relates to a sheet feeding apparatus used with an image forming system such as a copying machine, laser beam printer and other equipment using sheets, and an image forming system using such sheet feeding apparatus. More particularly, the present invention relates to a sheet feeding apparatus wherein sheets stacked on a sheet stacking plate are separated one by one by means of a sheet separating and supplying means comprising sheet supply rollers and separating members urged against each other with a predetermined pressure and the separate sheet is fed by means of a conveying means disposed at a downstream side of the separating and supplying means in a sheet feeding direction, and an image forming system using such sheet feeding apparatus.
The sheet used in the sheet feeding apparatus may be a transfer sheet, recording sheet, printing sheet, OHP sheet, original, post card, envelope, card, film or the like. Material of the sheet is not limited to paper, but may be plastic, metal, cloth or the like. Further, not only a single sheet, but also a multi-sheet such as slips or pamphlets bound together.
2. Related Background Art
A conventional sheet feeding apparatus of the above-mentioned kind is schematically shown in FIG. 12. In FIG. 12, the reference numeral 21 denotes a sheet supply shaft; 22 denotes an eccentric sheet supply roller (feeding member) integrally formed with the sheet supply shaft substantially at its control portion; 23 denotes a pair of idler rollers rotatably and concentrically mounted on the sheet supply shaft 21 on both sides of the eccentric sheet supply roller (only one of which is shown in FIG. 12); 24 denotes a pair of eccentric cams integrally formed with the sheet supply shaft 21 at both its ends (only one of which is shown in FIG. 12). The pair of eccentric cams 24 have the same configuration and are disposed on both ends of the sheet supply shaft 21 with the same phase, and are positioned in correspondence with upwardly directing projections 28a formed on both leading ends of a sheet stacking plate 28 which will be described later. The eccentric sheet supply roller 22 has a configuration substantially the same as those of the eccentric cams 24, and is disposed on the sheet supply shaft 21 with the phase offset from the eccentric cams 24 by generally 180 degrees. A larger diameter portion 22a of the sheet supply roller has a diameter slightly larger than those of the idler rollers 23 so that an outer peripheral surface of the larger diameter portion 22a is protruded outwardly from those of the idler rollers 23.
The reference numeral 26 denotes a separating pad disposed at a downstream side of the idler rollers 23; 25 denotes a separating pad receiving member; and 27 denotes a bias spring for urging the pad receiving member upwardly. The separating pad 26 is urged against lower surfaces of the idler rollers 23 by means of the bias spring 27. The reference numeral 28 denotes the above-mentioned sheet stacking plate a rear end of which is rotatably supported on a shaft 30 for pivotal movement around the shaft 30 in an up-and-down direction; and 29 denotes a bias spring for urging the sheet stacking plate upwardly so that the sheet stacking plate 28 is always biased for upward pivotal movement around the shaft 30.
FIG. 12 shows a sheet supply waiting condition in which the eccentric cams 24 are oriented leftwardly and downwardly so that larger diameter portions of the cams urge the projections 28a of the sheet stacking plate 28 downwardly, thereby rocking the sheet stacking plate 28 downwardly around the shaft 30 in opposition to the bias spring 29. In this condition, the separating pad 26 is abutted against the lower surfaces of the idler rollers 23, and the eccentric sheet supply roller 22 is oriented rightwardly and upwardly with the phase offset from the eccentric cams 24 by generally 180 degrees. Sheets P are stacked on the sheet stacking plate 28.
In the sheet supply waiting condition shown in FIG. 12, when a sheet supply start signal is emitted, the sheet supply shaft 21 is rotated by one revolution in a clockwise direction via a drive means and a one-revolution clutch mechanism (both not shown), so that the eccentric cams 24 and the eccentric sheet supply roller 22 are also rotated by one revolution, together with the sheet supply shaft 21.
(1) At an initial phase of this one revolution, the downward urging force of the eccentric cams 24 against the sheet stacking plate 28 is released, with the result that the sheet stacking plate is rocked upwardly by the spring 29, thereby abutting a leading end portion of the sheet stack P rested on the sheet stacking plate against the lower surfaces of the idler rollers 23. PA0 (2) During a further rotation in the one revolution, a sheet supply start end 22b of the larger diameter portion 22a of the eccentric sheet supply roller 22 is urged against the leading end portion of the sheet stack P rested on the sheet stacking plate 28, as shown in FIG. 13, thereby applying a sheet feeding force to an uppermost sheet on the sheet stack, with the result that a further rotation of the eccentric sheet supply roller 22 causes the uppermost sheet to be fed toward the separating pad 26 and be passed between the separating pad 26 and the eccentric sheet supply roller 22 together with the larger diameter portion 22a. In this case, the other sheets are prevented from passing between the separating pad 26 and the eccentric sheet supply roller 22 by the separating pad 26, with the result that only the uppermost sheet contacting the larger diameter portion 22a of the eccentric sheet supply roller 22 can be separated from the other sheets. PA0 (3) When a sheet supply finish end 22c of the larger diameter portion 22a of the eccentric sheet supply roller 22 has been passed through the separating pad 26 as shown in FIG. 14, the separated uppermost sheet is guided by a guide 31 to reach a nip between a pair of register rollers 32 now stopped. During a further sheet feeding operation, a loop is formed in the sheet between the paired register rollers 32 and a contacting point between the separating pad 26 and the eccentric sheet supply roller 22. Due to the reaction force from the loop, a leading end of the sheet is abutted against the nip line of the paired register rollers 32 along the whole length of the leading end, thereby performing the registration of the sheet. After the sheet supply finish end 22c of the larger diameter portion 22a of the eccentric sheet supply roller 22 has passed through the separating pad 26, the sheet feeding force does not act on the sheet, so that the sheet is maintained to be pinched between the idler rollers 23 and the separating pad 26. PA0 (4) After the registration of the sheet by the register rollers 32 has finished, at a predetermined control timing, the register rollers 32 are rotated to feed the sheet. In this sheet feeding operation, the loop in the sheet is firstly eliminated first, and then, the sheet is subjected to a pulling force, so that the sheet is pulled off from the contacting portions between the idler rollers 23 and the separating pad 26 (FIG. 15). The idler rollers are rotatingly driven by the pulled sheet. PA0 (5) When one revolution of the sheet supply shaft 21 is finished, the eccentric cams 24 again urge the sheet stacking plate 28 downwardly in opposition to the spring 29, and the eccentric cams 24 and the eccentric sheet supply roller 22 are returned to the sheet supply waiting condition shown in FIG. 12. At this point, even if a trailing end of the sheet (fed by the register rollers 32) has not yet been passed through the contacting portions between the idler rollers 23 and the separating pad 26, the sheet continues to be fed. When the trailing end of the sheet has just passed through the contacting portions between the separating pad 26 and the idler rollers 23, these rollers 23 are stopped. Further, when the trailing end of the sheet passes through the register rollers 32, the latter is also stopped, with the result that the sheet feeding apparatus is maintained to the sheet supply waiting condition until the next sheet supply start signal is emitted.
The above-mentioned sheet feeding apparatus has the following problem. That is to say, as mentioned in the above item (4), after the separated sheet is registered by the paired register rollers 32 and is fed by these rollers, the sheet (after elimination of the loop) is pulled in opposition to a pinching force from the contacting portions between the idler rollers 23 and the separating pad 26. The pinching force acts on the sheet as a back tension. The back tension is determined by the pressure and coefficient of friction of the separating pad 26, and accordingly, when the pressure of the separating pad is increased due to the insufficient manufacturing accuracy of the pad or when the coefficient of friction of the separating pad is increased in accordance with the circumstances, the back tension is also increased. As a result, the trailing end of the sheet after registration is pulled back, thus deteriorating the registration of the sheet P. Thus, when such sheet feeding apparatus is used with an image forming system, an image quality is worsened, and/or the discrepancy in image occurs, thus leading to the detrimental reduction in the image quality.
It is considered that, when the sheet after registration is fed by the register rollers 32, the idler rollers 23 and the separating pad 26 are separated from each other to release the pinching force, thereby preventing the back tension from acting on the sheet. However, if the idler rollers 23 and the separating pad 26 are separated from each other, a next sheet will be adhered to the fed sheet due to the electrostatic force and the like, thus causing the following double feed.